From: Homebrew_PCBs@yahoogroups.com [mailto:Homebrew_PCBs@yahoogroups.com]
Sent: Sunday, November 06, 2016 6:54 AM
To: Homebrew_PCBs@yahoogroups.com
Subject: Re: [Homebrew_PCBs] DIY Gold Fingers?
Silver migration is nothing new; it's been known to those in the antique radio restoration business for many years in the form of "thunderstorming". In ye olde days, IF transformers for tube-type AM radios frequently were roughly tuned by small mica compression capacitors (The capacitors only put the IF transformers in the general frequency range; final tuning was done by tuning the ferrite slug in the coils). The mica slabs were silver plated; the mica being the insulator and the silver plating being the conductors for the capacitors.
After years of aging, the silver plating would migrate toward the edges of the mica slabs, eventually causing a partial short. Before the short caused a permanent failure, the current leakage across the capacitor would cause a noise in the receiver, noted at the speaker as sounding like the radio was being affected by a distant electrical thunderstorm; thus the term "thunderstorming"
Of course, the only cure was to replace the internal mica capacitors with new modern mica capacitors.
Dave M
---In Homebrew_PCBs@yahoogroups.com, <hrconsult@...> wrote :
The paper kinda, sorta, gets around to mentioning silver migration at the very end. The is a very important failure mechanism for silver plated traces that have a DC voltage difference between them. The paper mentions that this occurs under specific conditions, but does not say what those conditions are. The conditions are easily met. A small potential difference between the conductors, and moisture. A film of water is not necessary, but just plain humidity. If you want to see this very dramatically, and quickly, put 5 volts between two silver plated conductors and bridge them with a drop of water. Watch under about 10X magnification. A fern like (dendritic) structure will grow out from the negative side toward the positive trace. If you don't limit the current, the the dendritic structure will "explode" when it hits the positive trace. If you limit the current enough, the short will be permanent, even after the water evaporates.
A good description is available here:
http://www.te.com/documentation/whitepapers/pdf/p313-89.pdf
This is a well known failure mechanism in the aerospace community.
Harvey
On 11/5/2016 8:44 PM, k5ess.nothdurft@... [Homebrew_PCBs] wrote:
I posted a file RE silver vs gold plating for contacts. I probably shouldn't take up file space for this so i will delete the file and just supply the link.
The Performance Implications of Silver as a Contact ... - TE Connectivity <https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0ahUKEwiEhubckpPQAhUHbiYKHQfMDY0QFggjMAE&url=http%3A%2F%2Fwww.te.com%2Fdocumentation%2Fwhitepapers%2Fpdf%2FPerformance_implications_Silver_Myers2009.pdf&usg=AFQjCNGVx3zCZqRRKj0FXGcohb0U-lT_Cw&cad=rja>
Mike N.